A synthetic resin container closure having tamper evident properties is applied to a container which has an external thread formed on the outer peripheral surface of a mouth-and-neck portion and an engaging jaw portion formed to be located below the external thread. As such a synthetic resin container closure, Patent Document 1 to be described below discloses a synthetic resin container closure including a body which has a top panel wall, and a cylindrical skirt wall extending downwardly from the peripheral edge of the top panel wall, and in which an internal thread to be screwed to the external thread of the mouth-and-neck portion is formed on the inner peripheral surface of the skirt wall; and a cylindrical tamper evident bottom portion which is connected to the skirt wall of the body via a plurality of breakable bridge portions arranged with spacing in a circumferential direction, and in which an engaging means to be engaged with the engaging jaw portion of the mouth-and-neck portion is disposed on the inner peripheral surface of the cylindrical tamper evident bottom portion. A protruding piece protruding downward is disposed on the skirt wall of the body. On both sides in the circumferential direction of the protruding piece, there are disposed unbroken connecting pieces which connect the skirt wall of the body and the tamper evident bottom portion and which each have a tilting portion whose inclination angle with respect to a vertical line is gradually decreased when the body is moved upward with respect to the tamper evident bottom portion.
In mounting the above-described container closure on the mouth-and-neck portion of the container to seal the mouth-and-neck portion, the container closure is fitted on the mouth-and-neck portion, and rotated in a closing direction to screw the internal thread of the container closure to the external thread of the mouth-and-neck portion. As the screwing proceeds, the container closure is lowered down the mouth-and-neck portion, and the engaging means locked to the tamper evident bottom portion elastically passes over the engaging jaw portion of the mouth-and-neck portion until it is engaged below the engaging jaw portion. In unsealing the mouth-and-neck portion, the container closure is rotated in an opening direction to release the screwing between the external thread of the mouth-and-neck portion and the internal thread of the container closure. Along with the release of the screwing, the body of the container closure is allowed to ascend relative to the mouth-and-neck portion. On the other hand, the tamper evident bottom portion is inhibited from ascending, because its engaging means is engaged with the engaging jaw portion of the mouth-and-neck portion. Thus, stress is caused to the breakable bridge portions, whereby the breakable bridge portions are broken. When the breakable bridge portions are broken, the body continues to ascend, and the body is spaced upward from the tamper evident bottom portion, the tilting portions of the unbroken connecting pieces have their inclination with respect to the vertical line gradually decreased accordingly. Even after the body is removed from the mouth-and-neck portion, the tamper evident bottom portion continues to be mounted on the mouth-and-neck portion, while the body continues to be connected to the tamper evident bottom portion via the unbroken connecting pieces. Consequently, the container closure is not separated from the container.